As solid-state imaging devices (image sensors) using photoelectric conversion elements detecting light and generating a charge, CMOS (complementary metal oxide semiconductor) image sensors have been put into practical use. CMOS image sensors have been widely applied as parts of digital cameras, video cameras, monitoring cameras, medical endoscopes, personal computers (PC), mobile phones and other portable terminals (mobile devices) and other various types of electronic apparatuses.
A CMOS image sensor has a floating diffusion (FD) amplifier having a photodiode (photoelectric conversion element) and an FD layer (FD) for each pixel. Reading is performed by selecting a certain row in a pixel array and simultaneously reading out the pixels in a column direction, that is, a column parallel output type is the mainstream.
Further, various circuits have been proposed for the pixel signal read-out (output) circuit of a column parallel output type CMOS image sensor. Among them, one of the most advanced circuits is a circuit which is provided with an analog-to-digital converter (ADC) for each column and extracts pixel signals as digital signals (for example, see Japanese Patent Publication No. 2005-276135 and Japanese Patent Publication No. 2005-295346).
In this column parallel ADC-mounting CMOS image sensor (column AD system CMOS image sensor), a comparator compares a so-called RAMP wave and the pixel signals and performs digital CDS by a later stage counter to thereby perform AD conversion.
In this type of CMOS image sensor, however, while high speed transfer of signals is possible, there is the disadvantage that a global shutter reading operation cannot be carried out.
Contrary to this, a digital pixel sensor in which an ADC including a comparator (and further a memory part) is arranged in each pixel to also enable realization of a global shutter for executing the start of exposure and end of exposure at the sane timings with respect to all pixels in the pixel array has been proposed (for example, see U.S. Pat. No. 7,164,114, B2, FIG. 4 and US Patent No. 2010/0181464, A1).